Poly brushes substrates

The two examples of adsorbed side chain substituted macromolecules, i.e., the poly(n-butyl acrylate) brush and the tris(p-undecyloxybenzyloxo) benzoate jacketed polystyrene, demonstrate two rather complementary aspects of the interaction of such molecules with a planar surface. In the first case the two-dimension to three-dimension transition results in a cooperative collapse of an extended coil conformation to a globule. The second case shows a rather high degree ordering with a distinct orientation of the backbone in the substrate plane. Combination of both effects and partial desorption can lead to a repta-tion-hke directed motion as depicted schematically in Fig. 36. 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

Poly(methyl methacrylate) with a variable degree of polymerization anchored to silica surfaces was synthesized following the room temperature ATRP polymerization scheme described earlier [45,46]. In the main part of Fig. 25 we plot the variation of the PMMA brush thickness after drying (measured by SE) as a function of the position on the substrate. Thickness increases continuously from one end of the substrate to the other. Since the density of polymerization initiators is (estimated to be 0.5 chains/nm ) uniform on the substrate, we ascribe the observed change in thickness to different lengths of polymer chains grown at various positions. 

Fig. 26 Dry thickness of poly(acryl amide) as a function of the position on the silica substrate prepared by slow ( ) and fast ( ) removal of the polymerization solution by utilizing the method depicted in Fig. 24. The inset shows the dry poly(acryl amide) thickness as a function of the polymerization time. Note that both data sets collapse on a single curve at short polymerization times. Regardless of the drain speed, the brush thickness increases linearly at short polymerization times and levels off at longer polymerization times. The latter behavior is associated with premature termination of the growing polymers...

Fig. 38 (Upper panel) Scanning force microscopy images of gold nanoparticles (diameter 17 nm) adsorbed along a surface-anchored poly(acryl amide) brush with a molecular weight gradient (Edge of each image = 1 p.m). (Lower panel) Dry thickness of poly(acryl amide) on the substrate before particle attachment (right, ) and particle number density profile (left, ). (Reproduced with permission from [140])...

In this review, synthesis of block copolymer brushes will be Hmited to the grafting-from method. Hussemann and coworkers [35] were one of the first groups to report copolymer brushes. They prepared the brushes on siUcate substrates using surface-initiated TEMPO-mediated radical polymerization. However, the copolymer brushes were not diblock copolymer brushes in a strict definition. The first block was PS, while the second block was a 1 1 random copolymer of styrene/MMA. Another early report was that of Maty-jaszewski and coworkers [36] who reported the synthesis of poly(styrene-h-ferf-butyl acrylate) brushes by atom transfer radical polymerization (ATRP). 

During the last 5 years, there have been several reports of multiblock copolymer brushes by the grafting-from method. The most common substrates are gold and silicon oxide layers but there have been reports of diblock brush formation on clay surfaces [37] and silicon-hydride surfaces [38]. Most of the newer reports have utilized ATRP [34,38-43] but there have been a couple of reports that utilized anionic polymerization [44, 45]. Zhao and co-workers [21,22] have used a combination of ATRP and nitroxide-mediated polymerization to prepare mixed poly(methyl methacrylate) (PMMA)Zpolystyrene (PS) brushes from a difunctional initiator. These Y-shaped brushes could be considered block copolymers that are surface immobilized at the block junction. 

Zhao B (2004) A combinatorial approach to study solvent-induced self-assembly of mixed poly (methyl methacrylate)/polystyrene brushes on planar silica substrates effect of relative grafting density. Langmuir 20 11748-11755... 

As the charge of the particles is in equilibrium with the other species in solution, the zeta potential depends on the pH as well. Figure 13 shows the zeta potential of some materials concerned by the CMP process, where SiOj stands for both the substrate and the fumed glass slurries PVA is the material used for the scrubber brushes (poly vinyl alcohol) SijN. is used as a polish stop layer and AI2O3 and CeOj represent the alumina and ceria slurries, respectively. 

Surface-initiated ATRP was applied not only on planer substrates but also on various kinds of flne particles. The latter systems will be reviewed separately in Sect. 5.1. Porous materials are also fascinating targets for chromatographic application making use of the unique structure and properties of high-density polymer brushes. Wirth et al. were the first to report the grafting of poly(acrylamide) (PAAm) on a porous silica gel [109,110]. 

The polymer pair PVPON and PMAA was later used by Sukhishvih and coworkers to prepare hydrogen-bonded multilayer capsules and to discuss the effect of the substrate charge on the first adsorbed layer of the weak poly(carboxyhc acid) and the subsequent film growth [257]. Chen and coworkers used the same concept to prepare composite thin films by a hydrogen bonding assembly of polymer brushes and PVPON [258]. Spherical poly-... 

In a first example, for a densely grafted PEL brush system positively charged quaternized poly-4-vinylpyridine brushes have been prepared by following a two step approach [2, 63, 64, 66]. In the first step a neutral poly-4-vinylpyridine monolayer is prepared and, subsequently, charges are introduced by a second, polymer-analogous quaternization step. The grafting density of the parental neutral brush is adjusted by varying the polymerization time (Fig. 12) [63, 64]. The substrates were planar silicon substrates in... 

Polyacrylic acid (PAA)—P2VP mixed brushes were prepared by a similar synthetic procedure, by grafting of carboxyl-terminated poly(ferf-butyl acrylate) (PtBuA) and P2VP. Afterwards, PtBuA was hydrolyzed in the presence of p-toluene sulfonic acid. The same strategy was employed to graft mixed PEL brushes on polymer surfaces. In this case plasma treatment was used to functionalize surface of polymer substrates. We introduced amino groups on the surface of PA-6 and PTFE by treatment of the polymer samples with NH3 plasma. Then the carboxyl terminated homopolymers were grafted step by step from the melt to the solid substrate via amide bonds. 

How adsorption of the side chains to a flat substrate effects the backbone conformation has been observed in further microscopic detail for brush molecules with a methacrylate backbone and poly-(n-butyl acrylate) side chains. These poly(/v-butyl acrylate) brushes were prepared by living radical grafting from a multifunctional macromolecular initiator.38 The synthetic approach allowed observation of the same batch of molecules without (macroinitiator) and with poly(n-butyl acrylate) side chains (brush). 

Wu, T. Gong, P. Szleifer, I. Vlcek, P. Subr V. Grenzer, J., Behavior of surface-anchored poly(acrylic acid) brushes with grafting density gradients on solid substrates 1. experiment, Macromolecules 2007, 40, 8756-8764... 

Several interesting new concepts for the design of CdSe nanocrystal based polymer solar cells have been introduced recently. Snaith et al. have infiltrated CdSe nanocrystals into polymer brushes and demonstrated EQEs of up to 50% [256]. In this case the poly(triphenylamine acrylate) (PTPAA) chains were directly grown from the substrate by a surface-initiated polymerization on tethered initiator sites (Fig. 58). The authors pronounced the wide applicability of this method for the design of nanocrystal-polymer functional blends [256]. 

Figure 4.4 Three-stage switching of poly[2-(methacryloyloxy)ethylphosphate] as reported for brushes grown by A TRP from gold substrates [6J.

Although the previously discussed methods are applicable specifically to polymer substrates, there are also strategies for surface functionalization with polymer brushes that work on a broad range of substrates. A general method that allows producing polymer brushes on silicon, gold, perfluorinated poly(ethylene-co-propylene), and poly (styrene) has been recently introduced [21]. The first step of the modification sequence was not sensitive to the substrate used. An... 

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